In baseball, softball, and similar games, an inning is the basic unit of play, consisting of two halves or frames, the "top" and the "bottom". In each half, one team bats until three outs are made, with the other team playing defense. A full baseball game is typically scheduled for nine innings, while softball games consist of seven innings, although this may be shortened due to weather or extended if the score is tied at the end of the scheduled innings. The use of the term inning in baseball and softball contrasts with cricket and rounders, in which the term is innings in both singular and plural.

Baseball is a bat-and-ball sport played between two teams of nine players each, taking turns batting and fielding. The game occurs over the course of several plays, with each play generally beginning when a player on the fielding team, called the pitcher, throws a ball that a player on the batting team, called the batter, tries to hit with a bat. The objective of the offensive team is to hit the ball into the field of play, away from the other team's players, allowing its players to run the bases, having them advance counter-clockwise around four bases to score what are called "runs". The objective of the defensive team is to prevent batters from becoming runners, and to prevent runners' advance around the bases. A run is scored when a runner legally advances around the bases in order and touches home plate.